a. The Field of the Invention
This invention relates to the field of integrated circuit manufacturing. In particular, the invention relates to concepts and implementation techniques for the quick and efficient design, correction and verification of masks utilized in the manufacture of integrated circuits.
b. Description of Related Art
In designing an integrated circuit (IC), engineers typically rely upon computer simulation tools to help create a circuit schematic design consisting of individual devices coupled together to perform a certain function. To actually fabricate this circuit in a semiconductor substrate the circuit must be translated into a physical representation, or layout, which itself can then be transferred onto the silicon surface. Again, computer aided design (CAD) tools assist layout designers in the task of translating the discrete circuit elements into shapes which will embody the devices themselves in the completed IC. These shapes make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on.
The software programs employed by these CAD systems are usually structured to function under a set of predetermined design rules in order to produce a functional circuit. Often, these rules are determined by certain processing and design limitations. For example, design rules may define the space tolerance between devices or interconnect lines so as to ensure that the devices or lines do not interact with one another in any unwanted manner. Design rule limitations are frequently referred to as critical dimensions. A critical dimension of a circuit is commonly defined as the smallest width of a line or the smallest space between two lines. Consequently, the critical dimension determines the overall size and density of the IC. In present IC technology, the smallest critical dimension for state-of-the-art circuits is approximately 0.25 microns for line widths and spacings.
Once the layout of the circuit has been created, the next step to manufacturing the integrated circuit (IC) is to transfer the layout onto a semiconductor substrate. Optical lithography is a well known process for transferring geometric shapes onto the surface of a silicon wafer. The optical lithography process generally begins with the formation of a photoresist layer on the top surface of a semiconductor wafer. A mask having fully light non-transmissive opaque regions, which are usually formed of chrome, and fully light transmissive clear regions, which are usually formed of quartz, is then positioned over the photoresist coated wafer. Light is then shone on the mask via a visible light source or an ultra-violet light source. The light is focused to generate a reduced mask image on the wafer typically using an optical lens system which contains one or several lenses, filters, and or mirrors. This light passes through the clear regions of the mask to expose the underlying photoresist layer, and is blocked by the opaque regions of the mask, leaving that underlying portion of the photoresist layer unexposed. The exposed photoresist layer is then developed, typically through chemical removal of the exposed/non-exposed regions of the photoresist layer. The end result is a semiconductor wafer coated with a photoresist layer exhibiting a desired pattern which defines the geometries, features, lines and shapes of that layer. This pattern can then be used for etching underlying regions of the wafer.
Besides the aforementioned design rules, the resolution value of the exposure tool used in optical lithography also places limits on the designers of integrated circuit layouts. The resolution for an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer. Currently, the resolution for most advanced optical exposure tools is around 0.25 micron. As the critical dimensions of the layout become smaller and approach the resolution value of the lithography equipment, the consistency between the mask and the actual layout pattern developed in the photoresist is significantly reduced. Specifically, it is observed that differences in pattern development of circuit features depends upon the proximity of the features to one another.
With these limitations on IC design in mind, we note the data describing an IC pattern is usually represented in a condensed hierarchical fashion such as in a GDS-II data file. At the higher levels of pattern representation hierarchy, features are represented in a conceptual manner. For instance, a memory array may be described as having a given cell repeated for a certain number of rows and columns. The next lower level in the hierarchy might describe the basic memory cell, comprised of subcells A and B. Finally, at the lowest level, the most primitive subcells contain geometric primitives-rectangles and polygons. In order to generate a physical mask, the hierarchical data must first be flattened, enumerating every geometric instance described in the hierarchy. Flattening of the hierarchy typically results in several orders of magnitude increase in the size of data storage required to represent the pattern.
Since flattening the hierarchy results in such a large increase in the size of the file representing a given IC design, it is desirable to flatten the hierarchy at the latest point in the manufacture of a mask, which, in the best case, is at the time the mask design is loaded into the EB machine prior to physical manufacture. Currently however, this flattening process takes place at an earlier stage in the production of masks for some complicated IC's. This is because the original mask design for a complicated IC is typically manipulated after the original design is completed in order to perform one of a number of operations on the design. These operations are performed because of the precision needed in the masks for complicated IC's as the critical dimensions of these IC's approach the resolution limits of optical lithography. Currently, these operations require some sort of flattening of the original design data in order to be performed—resulting in an earlier than desired flattening of the design data.
These operations include the performance of logical operations, the generation of optical proximity corrections, the generation of phase shifting masks, and the design rule checking of masks that have undergone these operations. For instance, since the physical mask making process may introduce known distortions in the mask that are dependent upon the particular EB machine being used, mask makers may use logical operations such as AND or NOT operations between design layers to generate new mask layers which compensate for these known distortions. Further, mask designers may generate sub-resolution optical proximity correction features for a mask to compensate for the proximity effects which occur when very closely spaced pattern features are lithographically transferred to a resist layer on a wafer. Similarly, mask designers may generate phase shifting masks to overcome the effect of resolution limits on achievable circuit critical dimensions. Currently, each of these operations requires a flattening of the original design data in order to be performed. Further, and more importantly, because they do not maintain the original true hierarchical data format of the mask design, it is extremely difficult and time consuming to verify currently known masks upon which one of the previously mentioned operations has been performed using conventional verification tools which require the same hierarchical data format as the original mask.
Therefore, what is desired is a method and apparatus for performing operations on integrated circuit mask designs that solve the aforementioned problems of currently existing systems.